1. Field of the Invention
The present invention relates to methods and systems for magnetic resonance (MR) imaging of moving parts of a patient with reduced motion artifacts, in particular to MR imaging protocols in which both the magnetic field gradients and the flip angles (.alpha.) for exciting nuclear magnetization are adaptively selected.
2. Description of the Related Art
Motion, notably due to or induced by cardiac or respiratory motions, is unavoidable in many clinical MR imaging situations. Without correction, MR images of moving parts of a patient are well-known to contain various confusing artifacts, such as ghost images, which can easily lead to clinical misinterpretation of the MR images.
In one known method for correcting such motion artifacts, after MR signals are collected, the displacement of the moving part is measured, such as by an MR navigator protocol. The collected MR data is then retrospectively discarded, i.e., not used for image reconstruction, if the subsequently measured displacement exceeds a threshold value. See, e.g., Sachs et al., 1994, Magnetic Resonance in Medicine 32:639-645.
However, this method, known as motion "gating", has the problem of lengthening data collection times in proportion to the fraction of time during which the displacement of the moving part exceeds the threshold value. Also the steady state of nuclear magnetization must be maintained. If the steady state is not maintained, MR signals measured when the displacement is below threshold will have varying signal strengths. Varying signal strengths introduce unpredictable modulations in k-space that are also well-known to lead to artifacts in the visible image.
An improvement to the gating method, referred to herein as motion-adaptive gating ("MAG"), attempts to shorten the data collection period by collecting MR data with less stringent boundary conditions than required by simple gating. According to this method, without significant loss of image quality, data generated from phase encoding gradients with larger time-integrals (stronger gradients) can be acquired at greater displacements, such as at greater respiratory or diaphragmatic displacements, as long as data generated from phase-encoding gradients with smaller time-integrals are acquired at smaller displacements. Then, it is only for a comparatively smaller fraction of time, when the displacement becomes very large, that no useable MR signals at all can be collected. The MAG method is described in U.S. patent application Ser. No. 08/795,119 filed Feb. 7, 1997, now U.S. Pat. No. 5,977,769.
Although the MAG method may reduce data collection times, it can also lead to increased artifacts due to increased, unpredictable signal-strength modulations in k-space. It will be immediately appreciated that in MAG the actual sequence of phase-encoding gradients can be highly disordered. Depending on the motion's course, MR signals adjacent in k-space can be acquired at unpredictable and widely different times during data acquisition. Unless the strength of the MR signals is carefully and smoothly controlled throughout data collection, modulations will be superimposed on k-space having an unknown structures. Random modulation components blurs the point spread function; periodic components produce ghosts, possibly leading to such misinterpretations as focal stenosis. The requisite careful control of MR signal strengths is, however, difficult and problematic.
Therefore, since both gating and, especially, motion-adaptive gating, although reducing motion artifacts, may introduce further artifacts due to uncontrolled and unpredictable modulation of signal strength in k-space, what is needed are simple and reliable MR methods and apparatus for also reducing or eliminating these further artifacts in order to achieve MR images with reduced motion artifacts in reduced times.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicants' invention of the invention subsequently claimed.